Volker Ossenkopf-Okada
Universität zu Köln
Is the concept of molecular clouds outdated?
In many models of the ISM giant molecular clouds (GMCs) are treated as well confined, static regions of high density providing the mass reservoir for star-formation. Focusing on the material visible in CO emission lines the discussion concentrates on the question whether GMCs form rather through cloud-cloud collisions or converging flows. However, this approach ignores that whatever we observe in molecular lines is just the "tip of the iceberg". The mass reservoir for cloud and star-formation always includes the whole iceberg. Ubiquitous filamentary, turbulent structures and substructures on all scales create a configuration where most material is located close to surfaces in relatively dilute, translucent cloud structures, exposed to the interstellar radiation field and thereby forming photon-dominated regions (PDRs). They provide a significant mass reservoir of material not visible in CO emission. [CII] observations allow us to trace cold material that is at the verge of turning molecular, so far invisible in CO. SOFIA/FEEDBACK observations show the interaction of velocity components with different molecular fractions, indicating a continuous transition from atomic to molecular material. Denser parts of a stream may be identified as individual clouds but SILCC simulations and HyGAL observations of light hydrides prove that the clouds are always embedded in surrounding transitional material. Some material can be traced in [CII] emission, but a large fraction of the CO-dark envelope of the clouds can only be traced through absorption. Absorption in hydrides can directly measure the molecular fraction but requires background sources. Alternatively, the combination of [CII] and [OI] foreground absorption and HI self-absorption (HISA) can be used to quantify the translucent accreting material. To really follow the process, a direct measurement of cold H2 through an heterodyne absorption measurements at 28µm would be needed. Currently, we therefore have no way of obtaining a reliable estimate of the mass reservoir for star-formation and thus the star-formation efficiency.
